Composite metal molding and method for manufacturing thereof

ABSTRACT

A compression molding which is high in both dimensional accuracy and mechanical strength is difficult to manufacture by a powder molding process. Especially, a molding including a soft magnetic material with high soft magnetic properties is difficult to manufacture. A composite metal molding according to the present invention includes metal particles and the carbide of a resin intervening among the particles. It is manufactured by coating metal particles with a resin, molding the prepared molding material under pressure into a predetermined shape, and heating the prepared pressurized preform to calcine the resin and weld mutually the particles. The carbide of the resin has a weight ratio of 0.001 to 2% to the metal particles when the particles have their proportion expressed as 100. The particles have a weld ratio of 10 to 80%. The particles preferably contain a soft magnetic material and the resin is preferably a furan resin.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite metal molding includingmetal particles which welded mutually and resin carbide interveningamong the metal particles, a method for manufacturing the compositemetal molding and an electromagnetic driving element having a yokeforming a magnetic circuit upon energization to a field coil.

2. Description of the Related Art

There is known a powder molding process in which metal particles arecompression molded in a mold with a resin used as a bonding material. Amolding made by the powder molding process has the advantage of beingvery close in dimensions and shape to the mold and basically notrequiring any post-work. Accordingly, the powder molding process can besaid to be a process which is effective for manufacturing mainly aproduct of an expensive material or a product which is difficult to makeby cutting work. The molding made by the powder molding process islimited in mechanical strength because of its structure including aresin as a bonding material among metal particles. Accordingly, themolding made by the powder molding process is often used as an elementfor which mechanical strength is not very important. For example, amolded magnetic member made by employing a rare earth magnet powder asmetal particles is adopted as e.g. a circular columnar rotor in a motor.The scope of its application is further expanded to include e.g. theyoke or stator of a motor, the yoke or transformer of an actuator in anoptical instrument and the core of a magnetic head which are all made byemploying a soft magnetic material as metal particles.

In order to improve the magnetic properties of a magnetic member moldedby employing a hard magnetic material as metal particles, it iseffective to apply as high a molding pressure as possible to bring themetal particles close together to realize a high magnetic flux density.It is also effective to heat a compression molding to harden the resinamong metal particles and then return the resin to ambient temperatureto make it undergo thermal contraction, as disclosed in Japanese PatentApplication Laid-open No. 7-176416 (1995). This makes it possible tobring the particles closer together and raise their coercive force owingto their thermal contraction strain to achieve an improved maximumenergy product.

There is also known a product made by heating a compression moldingextremely to weld metal particles together, while removing a bondingmaterial completely in a degreasing step as according to a powdermetallurgical sintering process, and thereafter sizing it to finish itinto a desired size and shape. However, a molded magnetic body made byemploying metal particles which are greatly affected by stress strain,such as a soft magnetic material, has its soft magnetic propertieslowered by processing strain resulting from post-processing, such assizing. It has also been likely that a molded magnetic body having anasymmetric complicated shape may not permit any such sizing. In order toovercome these inconveniences, Japanese Patent Application Laid-open No.6-017103 (1994) proposes a method in which a sintered product ismanufactured accurately by inserting a correcting member in a hollowcompact and placing the compact on a plate having a protruding orrecessed mark to support it in its end surface.

The epoxy resin which is usually employed as a bonding material for abonded magnet can withstand a temperature of only, say, 300° C., andcannot withstand the temperature for the stress-relief annealing of asoft magnetic material which may be as high as about 1,000° C. Anyattempt to heat for stress relieving a molded magnetic body made bybonding a soft magnetic material with an epoxy resin causes the epoxyresin to foam or disappear and lowers its strength and dimensionalaccuracy seriously. It will be possible to use water glass or a siliconeresin as a bonding material having higher heat resistance than the epoxyresin, but as they have only a very low force for bonding metalparticles, a large amount of bonding material is required for producinga desired bonding force. Moreover, the use of a large amount of bondingmaterial forms an enlarged clearance among metal particles making itimpossible to realize a high magnetic flux density, and resulting in amolded magnetic body of low magnetic properties.

In order to raise the mechanical strength of a compression molding madeby powder molding, it is effective to raise the density of its materialand it is, therefore, necessary to employ a higher molding pressure.However, if the compression molding of a material having its magneticproperties lowered by stress and strain, such as a soft magneticmaterial, employs an increased molding pressure to raise its softmagnetic properties, the stress and strain bearing on the soft magneticmaterial itself increase and thereby lower its soft magnetic properties.

While it is possible to form an irregularly shaped member by theelectrical discharge machining or wire-cut electrical dischargemachining of a metallic material, it is too low in mass productivity forpractical use. While it is also possible to form an irregularly shapedmember by a powder metallurgical method, the scattering of a bondingmaterial during sintering or the welding of particles causes so large adimensional change in a sintered product that a long time is requiredfor its post treatment. Even if a product of a soft magnetic materialmay allow sizing for its shape correction, its processing strain lowersits soft magnetic properties seriously.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a composite metalmolding containing a soft magnetic material and having good dimensionalaccuracy and mechanical strength and a method for manufacturing thecomposite metal molding. It is one of the other objects of the presentinvention to provide an electromagnetic drive element being able toemploy the composite metal molding as a yoke for a motor using a movingmagnet.

A first aspect of the present invention is concerned with a compositemetal molding including metal particles which welded mutually and resincarbide intervening among the metal particles, the metal particlecontaining a soft magnetic material.

In the present invention, a furan resin is preferably used as a bondingmaterial for metal particles, since when a pressurized preform isheated, the furan resin is not burned away completely, but remains to anadequate extent enabling the molding to avoid any large dimensionalchange as any molding of the powder metallurgical sintering processmakes. When the pressurized preform is heated, the metal particles arewelded mutually to an adequate extent enabling a composite metal moldingof improved mechanical strength to be manufactured.

According to the composite metal molding of the present invention, themolding made by heating a pressurized preform hardly differs indimensions or shape from the pressurized preform, but provides a moldingof high dimensional accuracy not calling for any post working, buthaving good soft magnetic properties. This is due to the presence ofresin carbide among the metal particles welded together.

In the composite metal molding according to the first aspect of thepresent invention, the resin carbide preferably have a weight ratio inthe range of 0.001 to 2% to the metal particles when the metal particleshave their proportion expressed as 100. Its weight ratio is morepreferably in the range of 0.003 to 1.5% and most preferably in therange of 0.005 to 1.0%. If the weight ratio of the resin carbide to themetal particles exceeds 2%, the molding is of lower density andmechanical strength and is of lower dimensional accuracy due to anincrease of carbonized resin and the gas gathering in the molding. It isof lower dimensional accuracy when the ratio is lower than 0.001%, too.The weight ratio of the resin carbide to the metal particles can bedetermined by infrared absorption spectroscopy after combustion.Basically, an increase in the weight ratio of the carbide brings about alowering in the density and mechanical strength of the composite metalmolding. On the other hand, a decrease in the weight ratio of thecarbide brings about a lowering in the dimensional accuracy of themolding. However, too high a weight ratio of the carbide also bringsabout a lower dimensional accuracy due to an increase of carbonizedresin and the gas gathering in the molding.

The metal particles containing a soft magnetic material preferably havea weld ratio in the range of 10 to 80% and more preferably in the rangeof 15 to 75%. A weld ratio exceeding 80% results in a molding of lowdimensional accuracy. On the other hand, a weld ratio which is lowerthan 10% results in a molding of low mechanical strength. It also failsto show good magnetic properties. The weld ratio R of the metalparticles is determined by grinding the surface of the composite metalmolding and measuring the total outer circumferential length L of themetal particles exposed in its surface and the length C of the weldedportion and is expressed as R=(2C/L) ×100.

The weight ratio of the resin carbide set in the range of 0.001 to 2% tothe metal particles taken to be 100 and the weld ratio of the metalparticles set in the range of 10 to 80% ensure the manufacture of acomposite metal molding of high dimensional accuracy. Moreover, themolding is high in mechanical strength and excellent in massproductivity.

Another simple method of determining the weld ratio is based on the factthat a higher weld degree of metal particles in a composite metalmolding results in a lower resistance thereof, and the weld ratio isdetermined by measuring the volume resistance of a bulk formed from thesame metal as a composite metal molding, dividing it by the volumeresistance of the molding and multiplying the result by 100.

Still another method is based on the fact that a higher weld degree ofmetal particles in a composite metal molding results in a lowerresistance thereof when the particles are of a soft magnetic material,and the weld ratio is determined by measuring the core loss of a bulkformed from the same metal as a composite metal molding and calculatingthe ratio of the core loss of the molding to the core loss of the bulktaken to be 100.

The value of the weld ratio R as herein adopted was obtained by the lastmethod utilizing the ratio in core loss, though a similar value can beobtained by any other method. This method is the easiest of all when thecore loss of the bulk of the same metal as the composite metal moldingis known.

The smaller the value of the weld ratio, the lower mechanical strengththe composite metal molding has, and the larger the value, the highermechanical strength the molding has. However, the larger the value ofthe weld ratio, the lower dimensional accuracy the molding tends tohave.

A second aspect of the present invention is concerned with a method formanufacturing a composite metal molding, including the step of coatingmetal particles containing a soft magnetic material with a resin toprepare a molding material, the step of molding the molding materialunder pressure into a predetermined shape to make a pressurized preform,and the step of heating the pressurized preform to calcine the resin andweld mutually the metal particles to make a composite metal moldingcontaining the metal particles and the carbide of the resin interveningamong the metal particles.

According to the method of the present invention, the method canmanufacture a composite metal molding having substantially the samedimensions and shape as the pressurized preform, since the heating ofthe pressurized preform does not burn away the resin completely, butallows it to remain in the carbide among the metal particles. As theadjoining metal particles are partially welded together, the compositemetal molding is high in dimensional accuracy and excellent inmechanical strength. It provides, among others, a molded magneticelement having good soft magnetic properties.

Although the timing for calcining the resin and the timing for weldingthe metal particles may differ from each other, they are preferably setat the same time to avoid any complication of the process. While thecomposite metal molding as manufactured can be used as a final molding,it can also be deburred, coated or treated for rust-proofing, ifrequired.

According to the present invention, the heating of the pressurizedpreform does not burn away the resin completely, but forms it intocarbide, and does not cause any such large dimensional change to thepressurized preform as is caused by the powder metallurgical sinteringprocess. The pressurized preform is preferably heated in an atmosphereof vacuum, reduction or inert gas.

Referring to the proportions of the metal particles and resin in themolding material, a higher proportion of the resin to the metalparticles makes it difficult to form a pressurized preform of highdensity and makes the metal particles difficult to weld together whenthe pressurized preform is heated, resulting in a composite metalmolding of low mechanical strength. On the other hand, a lowerproportion of the resin gives a pressurized preform of low strengthwhich is easily broken, and allows the welding of the metal particles toproceed extremely when it is heated, resulting in a composite metalmolding differing greatly in dimensions. Accordingly, the proportion ofthe resin containing additives, such as an acid catalyst, to the metalparticles in the molding material is preferably in the range of 0.1 to10 parts by mass to 100 parts by mass of metal particles when the resinis a furan resin. It is more preferably in the range of 0.3 to 5 partsby mass and most preferably in the range of 0.5 to 3 parts by mass.

The molding material consists mainly of the metal particles and resin.However, it preferably further contains 0.01 to 1 parts by mass of asolid lubricant selected from e.g. metallic soaps, higher fatty acids,talc, molybdenum disulfide and fluorocarbons to make a pressurizedpreform of high density and facilitate its removal from the mold. Theaddition of a solid lubricant in a large quantity is not desirable,since it usually lowers the mechanical strength of the pressurizedpreform. However, there is no particular limitation in the use of asolid lubricant which can reduce any friction force between the moldingmaterial and the mold and facilitate its movement in the mold to make apressurized preform of high density and is easily removable from thepressurized preform by gasification when the pressurized preform isheated to make a composite metal molding. A preferred solid lubricantsatisfying such requirements is a metallic soap and more preferably zincstearate.

In the method for manufacturing the composite metal molding according tothe second aspect of the present invention, the metal particles maycontain Fe, Ni, Co and a soft magnetic material composed of a softmagnetic alloy consisting mainly of any such element or a mixture ofsuch alloys. The step of heating the pressurized preform to make acomposite metal molding preferably includes the step of annealing thesoft magnetic material to remove its internal strain. This makes itpossible to obtain a molded magnetic body having still better softmagnetic properties.

The pressurized preform is preferably heated at a temperature enablingthe resin to be calcined without being burnt away and enabling the metalparticles to be welded together in an adequate weld ratio. Thetemperature depends on the amount of the resin forming a part of thepressurized preform, the kind of metal particles employed, etc. When itsheating temperature is low, it is generally the case that the removal ofstrain from the metal particles becomes insufficient, though thecalcination of the resin brings about only a small dimensional change inthe pressurized preform, or the composite metal molding. As a result,the metal particles fail to be welded mutually satisfactorily and make amolding of high mechanical strength and its soft magnetic properties arenot improved when the metal particles are of a soft magnetic material.

On the contrary, a high heating temperature causes the resin to undergoexcessive carbonization and eventually burn away, resulting in acomposite metal molding which has undergone an excessively largedimensional change leading to an undesirable phenomenon, though a highweld ratio of metal particles may give a higher mechanical strength.Therefore, the step of heating the pressurized preform is effective toinclude heating the pressurized preform at a temperature in the range of500° C. to 1,000° C. when the thermosetting resin is a furan resin. Ifits heating temperature exceeds 1,000° C., the molding is of lowerdimensional accuracy, though higher in mechanical strength. If it islower than 500° C., the metal particles fail to be relieved from straineffectively and welded together satisfactorily, resulting in a moldingwhich is low in mechanical strength and fails to show good mechanicalproperties.

Although there is no particular limitation as to the resin which can beemployed for the purpose of the present invention if it has a high forcefor bonding the metal particles without foaming or burning away whenheated, a thermosetting resin is preferred and a furan resin is, amongothers, preferred, as stated before. The furan resin is a general namefor the resins having furan rings, and it is possible to use a furfurylalcohol-furfural co-condensation type, furfuryl alcohol type orfurfural-phenol co-condensation type resin. Of course, it is alsopossible to use a furfural-ketone, furfuryl alcohol-urea, or furfurylalcohol-phenol co-condensation type resin. It is preferable to use anorganic or inorganic acid catalyst when curing the furan resin underheat. The amount of the acid catalyst added to the furan resin ispreferably from 0.001 to 10 parts by mass for 100 parts by mass of furanresin to ensure the curing of the resin and avoid any effects that thecatalyst may exert on the metal particles.

A third aspect of the present invention is concerned with anelectromagnetic driving element including a permanent magnet and a yokewhich formed a magnetic circuit upon energization to a field coil,wherein the yoke is the composite metal molding according to the firstaspect of the present invention or the composite metal moldingmanufactured by the method according to the second aspect of the presentinvention.

It is possible to obtain an electromagnetic drive element for e.g. alight amounts control device, or a motor having steady magneticproperties by using the composite metal molding according to the presentinvention as a yoke for an electromagnetic drive element including apermanent magnet and the yoke forming a magnetic circuit uponenergization to a field coil.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a light amounts control devicefor an optical instrument according to an embodiment of the presentinvention;

FIG. 2 is a scanning electron micrograph showing in an enlarged way theinternal structure of a yoke manufactured by the method according to thepresent invention and shown in FIG. 1;

FIG. 3 is a schematic diagram showing in a typical pattern the internalstructure of the yoke shown in FIG. 2;

FIG. 4 is a scanning electron micrograph of a positioning protrusion onthe yoke manufactured by the method according to the present inventionand shown in FIG. 1;

FIG. 5 is an exploded perspective view of a shutter element for a cameraaccording to another embodiment of the present invention;

FIG. 6 is a sectional view of a spindle motor according to still anotherembodiment of the present invention;

FIG. 7 is a top plan view of the yoke portion of the spindle motor shownin FIG. 6;

FIG. 8 is a sectional view of a cylindrical stepping motor according tostill another embodiment of the present invention;

FIG. 9 is an exploded perspective view of the cylindrical stepping motorshown in FIG. 8;

FIG. 10 is a sectional view of a different cylindrical stepping motoraccording to still another embodiment of the present invention; and

FIG. 11 is a perspective view of the yoke portion of the cylindricalstepping motor shown in FIG. 10.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of the present invention in which it is applied to anelectromagnetic drive element for a light amounts control device for anoptical instrument will now be described in detail with reference toFIG. 1 showing an exploded perspective view thereof. It is, however, tobe understood that the present invention is not limited to any suchembodiment thereof, but will permit any variation or modificationcovered by the concept of the present invention as claimed in the claimsand will naturally be applicable to any other art belonging to thespirit of the present invention.

The light control element 10 for an optical instrument which is shown inFIG. 1 has a pair of aperture diaphragms 11 and 12 comprising bladesaccording to the present invention and supported movably in mutuallyopposite directions by the rotation of a link arm 13 to vary the openingof an aperture defined by the aperture diaphragms 11 and 12. An NDfilter 14 is fixed to one of the aperture diaphragms 11 for preventingthe passage of an excessive amount of light which cannot be controlledby the aperture. The aperture diaphragms 11 and 12 are supportedreciprocally on a case 16 having an opening 15 for a path of light andhave slots 17 formed at their base ends, respectively, and eachconnected with one of pins 18 formed at the opposite ends, respectively,of the link arm 13. A shaft 20 projects from a cylindrical rotor magnet19 so magnetized as to have two poles along two halves, respectively, ofits circumference and has one end joined integrally to the middle of thelink arm 13. The rotor magnet 19 is attached to a bottom board 21rotatably by a bearing embedded in the bottom board 21, but not shown.The rotor magnet 19 has another end supported rotatably by a bearing notshown on a cap member 23 fitted on the ends of a pair of brackets 22projecting from the bottom board 21.

The bottom board 21 having an opening 24 formed for the path of lighthas a stopper portion projecting for abutting on the link arm 13 todefine its limits of rotation, though not shown. A cylindrical yoke 25surrounding the rotor magnet 19 in a spaced relation thereto has a pairof positioning protrusions 26 formed on its inner surface and eachengaged in a fitting hole 27 formed in the corresponding one of theelastically deformable brackets 22 of the bottom board 21 to be therebyunited with the corresponding bracket 22 of the bottom board 21. Theyoke 25 is formed from a soft magnetic material and forms with the rotormagnet 19 a magnetic circuit, or a moving magnet according to thepresent invention. A driving coil 28 for driving the rotor magnet 19 anda damping coil 29 for generating a back electromotive force proportionalto the rotating speed of the rotor magnet 19 to use it for controllingthe rotation of the rotor magnet 19 are situated on the diametricallyopposite side of the yoke 25 from each other. The driving and dampingcoils 28 and 29 are fixed to the yoke 25 with an adhesive tape 30 andconnected to a printed circuit board 31 for receiving a signal fromoutside. The driving and damping coils 28 and 29 are so attached to theyoke 25 as to face each other in a direction perpendicular to that inwhich the positioning protrusions 26 of the yoke 25 face each other. Thepositioning protrusions 26 have also the function of setting themagnetically stable position of the detent torque of the rotor magnet19. Therefore, it is possible to drive the aperture diaphragms 11 and 12and hold the aperture in its closed position by attracting the rotormagnet 19 magnetically when the supply of an electric current to thedriving coil 28 is interrupted.

The yoke 25 in the light amounts control element 10 is formed from acomposite metal molding according to the present invention and a methodfor its manufacture will now be described by way of example.

An iron powder-based soft magnetic composite, Somaloy 500 (trade name ofHöganäs AB, Sweden) was prepared as metal particles. A furan resin,VF303 (trade name of Hitachi Kasei Kogyo Co., Ltd.) was prepared as aresin, and A3 (trade name of Hitachi Kasei Kogyo Co., Ltd.) as an acidcatalyst therefor. A mixture of 100 parts by mass of furan resin and 1part by mass of acid catalyst was diluted with acetone, and its dilutionadjusted in viscosity was sprinkled on floating iron particles to coatthe iron particle surfaces with the furan resin containing the acidcatalyst upon evaporation of the acetone used as the solvent to therebyproduce a molding material. Zinc stearate, a common industrial reagent,was employed as a solid lubricant and 0.5 part by mass thereof was addeduniformly to the molding material and the molding material was chargedinto a mold by one second of vibration with a frequency of 120 Hz and avibrating force of 20 N by a piston vibrator. Then, a predeterminedpressure was applied to the molding material in a direction parallel tothe axis of the yoke 25 to make a pressurized preform.

The mold employed by the process under description had an insidediameter of 6.000 mm and its center pin had an outside diameter of 5.200mm and had one or two semicircular grooves formed in its outerperipheral surface to form a positioning protrusion or protrusions 26.

The pressurized preform was released from the mold, placed on a flatceramic plate and heated at 180° C. for an hour to have the furan resincured. While a vacuum of 10⁻³ Pa was maintained, it was heated at 600°C. for an hour so that the furan resin might be decomposed and removedas unnecessary gas. Its heating temperature was raised from 850° C. to1,000° C., held for an hour in a hydrogen reducing atmosphere forstrain-relief annealing and cooled to ambient temperature to yield ayoke 25 as a composite metal molding.

The powder metallurgical sintering of an iron-based metal usuallyemploys a temperature of 1,100° C. to 1,300° C. for welding metalparticles together. A weld ratio of 80% or higher is usually achieved togive a sintered molding of high mechanical strength, but a largedimensional change occurs to an article having a small thickness likethe molding of the process under description. When a plurality ofarticles are heat treated in contact with each other, their welding toeach other occurs with the carbonization of the furan resin and thewelding of the metal particles and brings about an extreme reduction inworking efficiency.

No satisfactory welding or removal of internal strain can be achieved atany temperature below 500° C. A composite metal molding can be formedeven by heating at 500° C., since the carbonization of a furan resinstarts at about 350° C.

The present invention can form a yoke having desired properties byselecting a firing temperature realizing a weld ratio allowing a certainamount of carbide to remain among metal particles, while not forming anyappreciably deformed molding, but forming a molding of high mechanicalstrength and magnetic properties.

The properties of the yokes 25 as produced are shown in Table 1 below.According to Example 1, a molding material was prepared by adding 1 partby mass of acid catalyst to 100 parts by mass of furan resin and adding0.6 part by mass of their mixture to 100 parts by mass of iron powderand a pressure of 3.6 tons/cm² was applied thereto to make a pressurizedpreform. According to Example 2, a molding material was prepared byadding 4.0 parts by mass of the above mixture to 100 parts by mass ofiron powder and a pressure of 3.6 tons/cm ² was applied thereto to makea pressurized preform. According to Example 3, a molding material wasprepared by adding 0.5 part by mass of the above mixture to 100 parts bymass of iron powder and a pressure of 10.0 tons/cm² was applied theretoto make a pressurized preform. According to Example 4, a moldingmaterial was prepared by adding 3.0 parts by mass of the above mixtureto 100 parts by mass of iron powder and a pressure of 10.2 tons/cm² wasapplied thereto to make a pressurized preform. According to Example 5, amolding material was prepared by adding 1.0 part by mass of the abovemixture to 100 parts by mass of iron powder and a pressure of 8.0tons/cm² was applied thereto to make a pressurized preform.

In order to ascertain the advantages of the present invention, apressurized preform according to Comparative Example 1 was made byemploying a molding pressure of 3.2 tons/cm² in Example 1. Likewise, apressurized preform according to Comparative Example 2 was made byemploying a molding pressure of 3.4 tons/cm² in Example 2, and apressurized preform according to Comparative Example 3 by employing amolding pressure of 15.6 tons/cm² in Example 3. According to ComparativeExample 4, a molding pressure of 12.5 tons/cm² was employed for amolding material prepared by adding 5.0 parts by mass of the abovemixture to 100 parts by mass of iron powder. According to ComparativeExample 5, the following was made. A molding material was prepared bydiluting 5 parts by weight of an elastomer, or a silicone resin,DY35-561A/B (trade name of Dow Corning Toray Co., Ltd.) with xylene andspraying its dilution on 100 parts by weight of iron powder as employedin Example 1. The molding material was molded with the same moldingpressure as employed in Example 5 and the molding was heated at 200° C.for four hours to have the silicone resin cured. Then, it was heated at600° C. for an hour, while a vacuum of 10⁻³ Pa was maintained, and itwas held for an hour in a hydrogen reducing atmosphere at a temperatureraised to 850° C. and was cooled to ambient temperature to give a yoke25 according to Comparative Example 5 as a composite metal molding.

TABLE 1 Weld Di- Radial Magnetic Residual ratio mensional crushingproper- carbon (%) (%) change strength ties Example 1 0.0010 10 ◯ ◯ ◯Example 2 1.9800 10 ◯ ◯ ◯ Example 3 0.0012 80 ◯ ◯ ◯ Example 4 1.9900 50◯ ◯ ◯ Example 5 0.0550 65 ◯ ◯ ◯ Comparative 0.0008 9 ◯ X X Example 1Comparative 2.0100 9 X X X Example 2 Comparative 0.0008 85 X ◯ X Example3 Comparative 2.0200 52 X ◯ X Example 4 Comparative — 4 X X X Example 5

The amount of residual carbon was determined by determining its ratio tothe metal element by infrared absorption spectroscopy after combustion.The weld ratio was determined by putting a winding on the yoke, applyinga magnetic field of 796 A/m to the yoke to determine its core loss at afrequency of 1 kHz and calculating its ratio to the core loss of a yokehaving the same shape as that of the corresponding Example, but formedfrom a bulk of pure iron when the latter was 100.

Every molding was graded as ◯ in dimensional change when the absolutevalue of an error in the outside diameter of the yoke 25 as comparedwith the inside diameter of the mold and the absolute value of an errorin the inside diameter of the yoke 25 as compared with the outsidediameter of the center pin in the mold were both smaller than 0.5%, andas X when they were not. When the absolute values of such errors aresmaller than 0.5%, the yoke 25 does not require any dimensionalcorrection.

The radial crushing strength was determined by employing the compressionmode of a material testing machine in accordance with the methodspecified by JIS Z2507. Every molding was graded as X when its radialcrushing strength was lower than 100 N/mm², and as ◯ when its radialcrushing strength was equal to, or higher than 100 N/mm², since anyarticle having a small wall thickness like each yoke 25 according to theembodiment under description had a sharply increasing tendency to crackor break when its radial crushing strength was lower than 100 N/mm².

The yokes 25 according to Examples 1 to 5 and Comparative Examples 1 to5 were each evaluated for magnetic properties by incorporating the yokein the light control device 10 as shown in FIG. 1 and checking theoperation of the aperture diaphragms 11 and 12 by interrupting thesupply of an electric current. Every yoke was graded as ◯ when theaperture diaphragms 11 and 12 closed the aperture smoothly, and as Xwhen they did not close the aperture smoothly, or at all.

The results shown in Table 1 confirm the high dimensional accuracy andstrength of each of the yokes 25 according to Examples 1 to 5 of thepresent invention as the soft magnetic member of the optical controldevice 10.

The yokes 25 according to Example 5 and Comparative Example 5 wereexamined for their magnetic flux density in a very weak magnetic fieldhaving an intensity of 796 A/m by putting 30 turns of primary andsecondary windings thereon and employing a direct-current magnetizationautomatic recording apparatus. The yoke 25 according to Example 5 showeda magnetic flux density of 1.05 T, while the yoke according toComparative Example 5 showed a magnetic flux density of 0.23 T. Theseresults confirmed that the yoke 25 according to the present inventionwas an excellent soft magnetic material having a very high magnetic fluxdensity in a very weak magnetic field.

On the contrary, the molding of Comparative Example 1 did not show anysatisfactorily high radial crushing strength, as its residual carboncontent and weld ratio were both low, nor was it satisfactory inmagnetic properties, either, though it was satisfactory in dimensionalaccuracy owing to its low weld ratio despite its residual carbon contentlower than 0.001%. The molding of Comparative Example 2 having aresidual carbon content higher than 2% showed an undesirably largedimensional change due to a large amount of carbonized resin and gasgathering in the molding. As its residual carbon content and weld ratiowere both low, it did not show any satisfactorily high radial crushingstrength, nor was it satisfactory in magnetic properties, either. Themolding of Comparative Example 3 was undesirably low in dimensionalaccuracy due to its weld ratio over 80% and its dimensional error wasapparently responsible for the failure of the aperture diaphragms towork smoothly. The molding of Comparative Example 4 having a residualcarbon content higher than 2% showed an undesirably big dimensionalerror due to a large amount of carbonized resin and gas gathering in themolding and it was apparently responsible for the failure of theaperture diaphragms to work smoothly.

The yoke 25 according to Example 5 was cut and had its cut surfacepolished smoothly and examined through a microscope. The result is shownin FIG. 2. FIG. 2 reveals metal particles welded together and voidsamong particles not welded together. FIG. 3 is a typical representationof the structure shown in FIG. 2. Carbide remains on the surfaces of thevoids shown in FIGS. 2 and 3 and restrains the welding together of ironparticles. FIG. 4 is a scanning electron micrograph taken after etchingaway iron particles with dilute nitric acid and revealing the carbide ofthe furan resin having a flat surface not eroded with the etchingsolution.

Thus, the control of the residual carbon content of the composite metalmolding in which the resin is calcined under heat and stays in carbideform makes it possible to manufacture a composite metal molding of highstrength even with a low weld ratio and of high accuracy by retainingits stability in shape and without causing any appreciable dimensionalchange. Moreover, the welding together of metal particles under heatimparts a very high level of mechanical strength to the molding. Whenthe metal particles are of a soft magnetic material, the removal ofinternal strain from the material under heat improves its soft magneticproperties remarkably.

Another form of embodiment of the present invention in which a compositemetal molding is employed as a shutter device for a camera will now bedescribed in detail with reference to FIG. 5 showing an explodedperspective view thereof. The shutter device 40 is of the so-called lensshutter and is held in a very narrow space in a lens barrel not shown.

A magnet rotor 43 formed from a cylindrical permanent magnet is heldbetween a casing 41 for the shutter device 40 and an upper cover 42 puton the casing 41. A yoke 44 surrounding the magnet rotor 43 in a spacedrelation thereto and forming therewith a moving magnet according to thepresent invention is held between the casing 41 and the upper cover 42,too. The magnet rotor 43 is so magnetized as to have two poles along twohalves, respectively, of its circumference. The magnet rotor 43 has arotary shaft 45 extending at one end through the casing 41 and through aslot 47 formed in a first shutter blade 46 at the base end thereof andfitted integrally in a connecting hole 49 formed in a second shutterblade 48 at the base end thereof. A swinging arm 50 fitted integrallyabout one end portion of the rotary shaft 45 of the magnet rotor 43 hasa driving pin 51 protruding from its distal end and fitted integrally ina connecting hole 52 formed in the first shutter blade 46 beside itsslot 47. Thus, the rotation of the magnet rotor 43 with its rotary shaft45 causes the second shutter blades 48 to rotate simultaneously, whilethere is a certain lag of time before the rotation of the first shutterblade 46. The rotary shaft 45 of the magnet rotor 43 has another endsupported rotatably by the upper cover 42.

The yoke 44 shaped substantially like a tuning fork is formed from asoft magnetic material and equipped with a winding 53 connected to apower source not shown. The control of the magnitude and direction of anelectric current supplied to the winding 53 makes it possible to selectthe operation of the shutter blades 46 and 48 as desired. That portionof the fork-shaped yoke 44 which faces the magnet rotor 43 is so shapedthat the magnet rotor 43 may be rotated in a direction closing theshutter blades 46 and 48 completely when the supply of an electriccurrent to the winding 53 is interrupted. As another function of theshutter, it is necessary for the shutter blades not to open uponreceiving any impact or the like, either, and that portion of the yokewhich faces the magnet rotor is particularly required to bedimensionally reliable. This point differentiates the yoke 44 from thesubstantially cylindrical yoke 25 as described before.

In order to obtain the stable performance of the shutter device 40, itis important to ensure the flatness of the yoke 44 forming the opposedmagnetic pole of the magnet rotor 43 and the dimensional accuracy of theclearance between the magnet rotor 43 and the yoke 44.

Therefore, an attempt was made to manufacture a yoke 44 by the processaccording to the present invention. A pressurized preform was formed byemploying a mold corresponding in shape to a yoke 44, the same moldingmaterial as in Example 5 and a molding pressure of 9.0 tons/cm². Theamount of the molding material and the degree to which the upper andlower punches of the mold were forced against the molding material wereso controlled that the molded yoke might have a diameter in its portionforming the opposed magnetic pole of the magnet rotor 43, 4,000 mm and athickness of 2.00 mm as released from the mold.

The pressurized preform was heated at 180° C. to have its furan resincured and then under the same conditions as in Examples 1 to 5 to yielda yoke according to Example 6 of the present invention as a compositemetal molding shaped substantially like a tuning fork.

According to Comparative Example 6, a flat sheet of pure iron having athickness of 1.00 mm was pressed into the shape of a yoke 44 and heattreated at 850° C. for an hour in a hydrogen atmosphere to make apressed yoke.

According to Comparative Example 7, a sintering material was prepared byadding 1.5 parts by weight of a polyamide resin as a binder to 100 partsby weight of an iron powder mentioned as Somaloy 500 before. It wasplaced in the same mold as had been employed in Example 6, and subjectedto a molding pressure of 9.0 tons/cm², and the amount of the sinteringmaterial and the degree to which the upper and lower punches of the moldwere forced against the sintering material were so controlled that themolding might have a thickness of 2.00 mm as released from the mold. Themolding was placed in a furnace held at 220° C. in an ambient atmosphereto have the polyamide resin degreased and was, then, subjected to onehour of sintering treatment at 1,100° C. in a hydrogen atmosphere tomake a sintered yoke.

(Evaluation for Dimensional Accuracy)

Ten yokes were prepared according to each of Example 6 and ComparativeExamples 6 and 7 and the average of the diameters of their portionsforming the opposed magnetic pole of magnet rotors 43 and the standarddeviation thereof were determined for comparing them in dimensionalaccuracy. In Comparative Example 6, however, the measurements were madeof only a single molding. The flatness of each yoke was calculated byplacing it on a base, measuring the maximum clearance therebetween,dividing it by the thickness of the yoke and multiplying the result by100 to express it in percentage. The value is the average of the tenyokes. The results are shown in Table 2.

TABLE 2 Average Standard Flatness diameter (mm) deviation (mm) (%)Example 6 3.998 0.006 0.25 Comparative 4.005 0.036 6.20 Example 6Comparative 3.922 0.056 2.13 Example 7

As is obvious from Table 2, the Example of the present invention showsthat it can make even for a fork-shaped member a composite metal moldinghaving only a very small degree of gap deformation at its end, a veryhigh level of flatness and so high a level of dimensional accuracy thatits dimensions are very close to those of the mold. The molding ofComparative Example 6 apparently had a great deal of uneven strainproduced by the pressing of pure iron and had its flatness lowered byheat treatment. The molding of Comparative Example 7 apparently had itsflatness lowered by the metal particles having a weld ratio of 80% orhigher as a result of heat treatment at 1,100° C.

(Evaluation for Shutter Performance)

A shutter device 40 made by putting a winding 53 on the yoke 44according to Example 6 of the present invention as shown in FIG. 5 wasput in a lens barrel not shown. A dropping impact test was conducted onthe shutter device 40 by keeping the shutter blades 46 and 48 closed bythe attractive force of the magnet rotor 43 formed from a permanentmagnet and the yoke 44 without supplying any driving current to thewinding 53, and by dropping the lens barrel from a height of 2.0 m. Asregards the performance required of the shutter device 40, the shutterblades 46 and 48 are required to remain closed not only when no electriccurrent is supplied, but also even after a given impact is applied toany optical instrument containing it. In all of the shutter devices 40containing the yokes according to the Example of the present inventionand the Comparative Examples, the shutter blades 46 and 48 were found toremain close when no electric current was supplied. As regards theresults of the dropping impact tests, the shutter blades 46 and 48 werefound to remain closed in the device containing the yoke according tothe present invention, but were found to open sometimes due to a weakholding force in the devices containing the yokes according to theComparative Examples.

(Material Strength Test)

A load was applied to each of the yokes according to Example 6 of thepresent invention and Comparative Examples 6 and 7 in the directionnarrowing its substantially channel-shaped end opening to determine itsbuckling strength. As regards the pressed yoke according to ComparativeExample 6, the test was conducted on two yokes laid one on the other.All of the yokes according to the present invention and the ComparativeExamples showed substantially the same buckling strength that wassufficiently high to withstand normal handling.

While the foregoing description has been of the case in which an ironpowder is used as a soft magnetic material, the present invention alsoenables the manufacture of a composite metal molding of high dimensionalstability and mechanical strength by using any other soft magneticmaterial, or any other appropriate metallic material, with a resin.

The molding according to the present invention can be employed not onlyas a yoke for an electromagnetic driving device for a light controldevice as described above, but also as a yoke for any otherelectromagnetic driving device, such as any of various kinds of motors.For example, the present invention is applicable to a yoke 55 having aplurality of radially extending coil holders 55 a as shown in FIG. 7which is used for a spindle motor 54 as shown in FIG. 6 which is usedfor e.g. a printer. The coil holders 55 a of the yoke 55 hold a coil 56wound thereon and an annular permanent magnet 57 surrounds the yoke 55and is rotatable with a spindle 58. The coil holders 55 a extendingradially from the cylindrical main body 55 b of the yoke 55 are ofimproved flatness and ensure a good accuracy of rotation as in the caseof Example 6.

FIGS. 8 and 10 show stepping motors 59 each having a permanent magnet 61fixed integrally to a spindle 60 and a pair of stators 63 each holding acoil 62 and fitted in a sleeve 64. The stators 63 have a high level ofdimensional accuracy which cannot be achieved by e.g. ordinary pressforming, since they are members of small wall thickness similar in shapeto the moldings of Examples 1 to 5 and having an annular array ofsaw-toothed projections 63 a as shown in FIGS. 9 and 11. Accordingly,the stepping motors 59 are excellent in stepping accuracy and torquecharacteristics.

Thus, the present invention provides as a composite metal molding any ofasymmetric or thin-walled articles of high dimensional accuracy andstrength including motor stators, transformers and magnetic head cores.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application Nos.2005-296480, filed Oct. 11, 2005, and 2006-259246, filed Sep. 25, 2006,which are hereby incorporated by reference herein in their entirety.

1. A composite metal molding including metal particles which weldedmutually and the carbide of a resin intervening among the metalparticles, the metal particles containing a soft magnetic material,wherein the carbide of the resin has a weight ratio in the range of0.001 to 2% to the metal particles when the metal particles have theirproportion expressed as 100, and wherein the metal particles have a weldratio of 10 to 80%.
 2. A molding as claimed in claim 1, wherein theresin is a furan resin.
 3. An electromagnetic driving element comprisinga permanent magnet and a yoke which formed a magnetic circuit uponenergization to a field coil, wherein the yoke is a composite metalmolding as claimed in claim 1.